......DE....FRGB..................................JDIM360 - Ver 2.9 (30 Jun 1998) 2100000/1 2100000/20668653EUROPEAN PATENT SPECIFICATIONB119980909EP94118346.9199411221996021019970414enenen24453/9419940222JP199809091998371995082319953419980909199837199801156 6H 02K 41/035 A 6H 02K 1/17 B 6G 11B 5/55 BdeMagnetische Schaltkreisstruktur in einer PlatteneinheitenMagnetic circuit structure in disk unitfrStructure d'un circuit magnétique dans une unité de disquesEP-A- 0 219 688EP-A- 0 544 537US-A- 4 661 729US-A- 5 168 184PATENT ABSTRACTS OF JAPAN vol. 017, no. 463 (E-1420) 24 August 1993 & JP-A-05 109 523 (MITSUBISHI MATERIALS CORP.) 30 April 1993PATENT ABSTRACTS OF JAPAN vol. 016, no. 018 (E-1155) 17 January 1992 & JP-A-03 235 651 (TOHO AEN KK) 21 October 19913Ogawa, Yoshinori, FUJITSU LIMITED1015, Kamikodanaka, Nakahara-kuKawasaki-shi, Kanagawa 211JPTochiyama, Kazunori, FUJITSU LIMITED1015, Kamikodanaka, Nakahara-kuKawasaki-shi, Kanagawa 211JPSato, Yoshihiro, FUJITSU SHINTER LIMITED1015, Kamikodanaka, Nakahara-kuKawasaki-shi, Kanagawa 211JPSuzuki, Hironori, FUJITSU SHINTER LIMITED1015, Kamikodanaka, Nakahara-kuKawasaki-shi, Kanagawa 211JPFUJITSU LIMITED002114601015, Kamikodanaka, Nakahara-kuKawasaki-shi, Kanagawa 211JPSeeger, Wolfgang, Dipl.-Phys.00011002SEEGER & SEEGER Patentanwälte & European Patent Attorneys Georg-Hager-Strasse 4081369 MünchenDEDEFRGB19970604199723 BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a magnetic circuit structure for driving an actuator assembly in a disk unit.

Description of the Related Art

In recent years, a magnetic disk unit as a kind of external storage for a computer has been desired to reduce in size and enlarge in capacity. In association with a reduction in size of the magnetic disk unit, it has been demanded to manufacture a compact, high-performance, low-cost magnetic circuit to be used in a voice coil motor (VCM) for driving an actuator assembly to be mounted in the magnetic disk unit.

In general, such a magnetic circuit is composed of a yoke and a plurality of magnets mounted on the yoke. In a large-sized magnetic disk unit such as a 20,32 cm (8 inch) disk unit or a 13,97 cm (5.5 inch) disk unit, the yoke is conventionally manufactured by casting or sintering. Further, the yoke and the magnets are connected together by bcnding in general. The bonding work requires much time and increases the cost, and a gas is generated from adhesive in the bonding work. The gas has an adverse effect on the magnetic heads. In particular, the adverse effect of the gas on thin-film magnetic heads for hight-density recording is large. A magnetic circuit for driving an actuator assembly in a disk unit according to the prior art is disclosed in document US-A-4 661 729.

The smaller the size of the magnetic disk unit, the smaller the height of the yoke. In the conventional large-sized magnetic disk unit, a yoke having a relatively large height is necessary, and it is therefore not realistic to manufacture the yoke by stacking a plurality of rolled steel plates adopted in the yoke of a rotary motor. Accordingly, the yoke in the large-sized magnetic disk unit is conventionally manufactured by casting or sintering as mentioned above. However, a yoke manufactured by such a method is relatively expensive because the yoke has a complex shape in general.

Further, the connection of the yoke and the magnets by bonding causes the problem that the gas generated from the adhesive adversely affects the magnetic heads as mentioned above. To cope with this problem, there has recently been proposed a magnetic circuit having such a structure that the magnets are attached by their magnetic forces to the yoke rather than the magnets are bonded to the yoke. However, the magnetic circuit having this structure requires means for preventing slip between the yoke and the magnets, causing a complexity in shape of the yoke and/or the magnets and an increase in cost of the magnetic circuit. As a result, the effect to be obtained by eliminating the bonding between the yoke and the magnets is not so large.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a magnetic circuit structure in a magnetic disk unit which can be manufactured simply at low costs.

It is another object of the present invention to provide a magnetic circuit structure in a magnetic disk unit which can prevent slip between the yoke and the magnets with a simple structure eliminating the bonding between the yoke and the magnets.

According to the present invention there is provided a magnetic circuit structure in a disk unit, comprising:

Preferably, a pair of plates for preventing vertical slip of the inner magnet are fixed to the upper surface and the lower surface of the inner yoke, and a pair of plates for preventing vertical slip of the outer magnet are fixed to the upper surface and the lower surface of the outer yoke.

As a modification, shallow grooves are formed at substantially central portions of upper and lower surfaces of the inner magnet and the outer magnet, and a plurality of projections engaging with the shallow grooves are formed on uppermost and lowermost rolled steel plates of the main yoke. These projections prevent the vertical slip of the inner magnet and the outer magnet.

According to an embodiment of the present invention there is provided a magnetic circuit structure in a disk unit which is characterized in that it contains a center yoke spaced a given distance from said inner yoke, said center yoke being also connected by said connecting portion.

According to the invention, these is also provided a magnetic disk unit having a magnetic circuit structure according to claim 1 and comprising:

In the magnetic circuit structure of the present invention, circumferential slip of the inner magnet and the outer magnet is prevented by the shoulders formed on the main yoke. Furthermore, vertical slip of the inner magnet and the outer magnet is prevented by the plural plates fixed to the upper surface and the lower surface of the main yoke. Accordingly, the magnets can be fixed to the main yoke with no occurrence of any slip without using adhesive.

The above objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Some preferred embodiments of the present invention will now be described in detail with reference to the drawings. Referring to Fig. 1, there is shown a perspective view of a magnetic disk unit mounting a magnetic circuit of the present invention therein. Reference numeral 12 denotes a housing (enclosure) composed of a base 14 and a cover 16. A spindle hub 18 to be rotatably driven by an inner hub motor not shown is provided on the base 14. A plurality of magnetic disks 20 and spacers not shown are alternately stacked and mounted on the spindle hub 18. That is, the plurality of magnetic disks 20 are regularly spaced from each other and are mounted on the spindle hub 18.

Reference numeral 22 denotes a rotary actuator assembly composed of an actuator arm assembly 26 and a magnetic circuit 28. As best shown in Fig. 2, the actuator arm assembly 26 is rotatably mounted on a shaft 24 fixed to the base 14, and includes a plurality of actuator arms 30 extending in one direction from the center of rotation and a pair of coil supporting members 34 and 36 extending in another direction opposite to the direction of extension of the actuator arms 30. A spring arm 33 supporting a magnetic head 32 is fixed to an end portion of each actuator arm 30. A coil 38 is supported by the pair of coil supporting members 34 and 36 and is inserted in two gaps of the magnetic circuit 28. The magnetic circuit 28 and the coil 38 constitute a voice coil motor 40. Reference numeral 42 denotes a flexible printed wiring board for taking out a signal from each magnetic head 32. The flexible printed wiring board 42 is fixed at its one end by a fixing member 44 and is electrically connected to a connector 46.

Referring again to Fig. 1, an annular packing assembly 48 is mounted on the base 14. The cover 16 is fixed to the base 14 by screws with the annular packing assembly 48 interposed therebetween, thus sealing the housing 12. Referring to Figs. 2 and 3, the magnetic circuit 28 includes a main yoke 50 formed by stacking a plurality of cold rolled steel plates 52 and connecting them together by caulking. An inner magnet 54 and an outer magnet 56 are attached by their magnetic forces to the main yoke 50. Reference numeral 58 denotes a side yoke attached to the main yoke 50 by the magnetic forces of the inner magnet 54 and the outer magnet 56.

The magnetic circuit 28 according to a first preferred embodiment of the present invention will now be described in more detail with reference to Figs. 4 and 5. A plurality of projections 52a are formed on an upper surface of each rolled steel plate 52, and a plurality of recesses 52b each having a shape corresponding to that of each projection 52a are formed on a lower surface of each rolled steel plate 52. The main yoke 50 is manufactured by stacking the plural rolled steel plates 52 in such a manner as to engage the projections 52a of each lower rolled steel plate 52 with the recesses 52b of each upper rolled steel plate 52, and then connecting the stacked rolled steel plates 52 together by caulking.

As best shown in Fig. 4, the main yoke 50 has an E-shaped configuration so that it is composed of an arcuate inner yoke 50a, an arcuate center yoke 50b, an arcuate outer yoke 50c, and a connecting portion 50d integrally connecting the inner yoke 50a, the center yoke 50b, and the outer yoke 50c. An outer circumferential surface of the inner yoke 50a is formed with a pair of shoulders 51 against which both circumferential ends of the inner magnet 54 abut, thereby preventing the inner magnet 54 from sliding in its circumferential direction. An inner circumferential surface of the outer yoke 50c is formed with a pair of shoulders 53 against which both circumferential ends of the outer magnet 56 abut, thereby preventing the outer magnet 56 from sliding in its circumferential direction.

As shown in Fig. 5, a pair of plates 60 and 62 for preventing the inner magnet 54 from sliding in its vertical direction are fixed by caulking to an upper surface and a lower surface of the inner yoke 50a. Similarly, a pair of plates 64 and 66 for preventing the outer magnet 56 from sliding in its vertical direction are fixed by caulking to an upper surface and a lower surface of the outer yoke 50c.

In Fig. 4, reference numerals 60a and 64a denote a plurality of caulked portions of the plates 60 and 64. An inner gap 55 is defined between the inner magnet 54 and the center yoke 50b, and an outer gap 57 is defined between the outer magnet 56 and the center yoke 50b. The coil 38 of the actuator arm assembly 26 is inserted in these gaps 55 and 57 to construct the voice coil motor 40. The plates 60 and 64 fixed to the upper surface of the main yoke 50 are formed from a spring steel plate, and the plates 62 and 66 fixed to the lower surface of the main yoke 50 are formed from a cold rolled steel plate.

As shown in Figs. 5 and 6, the plate 60 formed from a spring steel plate has a front bent portion 60b for downward pressing the inner magnet 54 to firmly fix it in the vertical direction. Similarly, the plate 64 also has a front bent portion 64b for downward pressing the outer magnet 56 to firmly fix it in the vertical direction.

In general, parts prone to be rusted, such as a yoke for a magnetic circuit, require to be subjected to perfect anticorrosion treatment. The main yoke 50 according to this preferred embodiment is subjected to anticorrosion treatment to be described below. First, the main yoke 50 formed by stacking the plural rolled steel plates 52 and connecting them together by caulking is immersed into a liquid resin, and evacuation is carried out to penetrate the resin between the rolled steel plates 52 adjacent to each other. Thereafter, the main yoke 50 is taken out from the liquid resin to cure the resin attached to the rolled steel plates 52. Then, excess resin attached to the surface of the main yoke 50 is removed by sand blasting. Thereafter, the surface of the main yoke 50 is plated with nickel. Such a series of treatment to the main yoke 50 improves the anticorrosion property of the main yoke 50.

Instead of removal of the excess resin from the surface of the main yoke 50 by sand blasting, a resin coating may be applied to the entire surface of the main yoke 50. As another anticorrosion treatment, steam treatment may be carried out to form an Fe3O4 coating having a strong anticorrosion property on the surface of each rolled steel plate 52. As another method, a brazing material may be layered between the rolled steel plates 52 adjacent to each other and then melted by heating to be tightly attached to each rolled steel plate 52.

Now, the structure of a magnetic circuit 28' according to a second preferred embodiment of the present invention will be described with reference to Figs. 7 to 9. In the following description of the second preferred embodiment, substantially the same components as those of the first preferred embodiment are denoted by the same reference numerals, and the explanation thereof will be omitted herein.

A main yoke 50 according to this preferred embodiment is manufactured by stacking a plurality of hot rolled steel plates 52' formed by pressing and then connecting them together by caulking. The main yoke 50 according to this preferred embodiment also has an E-shaped configuration as similar to that in the first preferred embodiment. As shown in Fig. 8, a shallow groove 54a is formed at a substantially central portion of an upper surface of an inner magnet 54, and a shallow groove 56a is also formed at a substantially central portion of an upper surface of an outer magnet 56.

An uppermost rolled steel plate 70 forming an uppermost layer of the main yoke 50 has two projections 70a and 70b respectively engaging with the grooves 54a and 56a, so that the inner magnet 54 and the outer magnet 56 are downward pressed by the projections 70a and 70b. Similarly, shallow grooves 54a and 56a are also formed at substantially central portions of lower surfaces of the inner magnet 54 and the outer magnet 56, respectively, and a lowermost rolled steel plate 72 forming a lowermost layer of the main yoke 50 has two projections 72a and 72b respectively engaging with the grooves 54a and 56a formed on the lower surfaces of the inner magnet 54 and the outer magnet 56 to upward press the inner magnet 54 and the outer magnet 56.

In this manner, the inner magnet 54 and the outer magnet 56 are pressed in the vertical opposite directions by the projections 70a and 70b of the uppermost rolled steel plate 70 and the projections 72a and 72b of the lowermost rolled steel plate 72, thereby preventing vertical slide of the inner magnet 54 and the outer magnet 56.

In Fig. 8, reference numerals 71 denote a plurality of caulked portions. The uppermost rolled steel plate 70 further has a pair of projections 70c for downward pressing a side yoke 58, and the lowermost rolled steel plate 72 also has a pair of projections 72c for upward pressing the side yoke 58. The depth of each of the shallow grooves 54a and 56a is equal to the thickness of each of the uppermost rolled steel plate 70 and the lowermost rolled steel plate 72. The main yoke 50 according to this preferred embodiment is also subjected to anticorrosion treatment similar to that mentioned in the first preferred embodiment, so as to improve the anticorrosion property.

In the magnetic circuit 28' according to the second preferred embodiment, an elastic sheet may be interposed between the upper groove 54a of the inner magnet 54 and the projection 70a of the uppermost rolled steel plate 70, and an elastic sheet may be interposed between the upper groove 56a of the outer magnet 56 and the projection 70b of the uppermost rolled steel plate 70. Similarly, an elastic sheet may be interposed between the lower groove 54a of the inner magnet 54 and the projection 72a of the lowermost rolled steel plate 72, and an elastic sheet may be interposed between the lower groove 56a of the outer magnet 56 and the projection 72b of the lowermost rolled steel plate 72. In this case, the inner magnet 54 and the outer magnet 56 can be more firmly fixed by caulking the projections 70a, 70b, 72a, and 72b with the elastic sheets interposed as mentioned above.

Referring to Fig. 10, there is shown a characteristic of each magnetic circuit mounted in the magnetic disk unit. In Fig. 10, a curve A corresponds to a conventional magnetic circuit having magnets bonded by adhesive; a curve B corresponds to the magnetic circuit according to the first preferred embodiment of the present invention; and a curve C corresponds to the magnetic circuit according to the second preferred embodiment of the present invention. Further, a represents an actuator acceleration, and i represents a coil current.

In the conventional magnetic circuit A, a force constant of the VCM is remarkably reduced on both sides. In the magnetic circuit B according to the first preferred embodiment of the present invention and the magnetic circuit C according to the second preferred embodiment of the present invention, a magnetic flux density at a magnetic flux concentrating portion of the yoke is relaxed to result in a slight increase in the force constant on both sides as compared with the conventional magnetic circuit A. In the curve C showing the characteristic of the second preferred embodiment, the magnetic flux density at a central portion 74 is large enough. Therefore, even if the height of the magnets is somewhat cut, an enough force constant can be ensured.

Now, a magnetic circuit according to a third preferred embodiment of the present invention in case of employing a U-shaped yoke will be described with reference to Figs. 11 and 12. Referring to Fig. 11, reference numeral 26' denotes an actuator assembly rotatably mounted on a shaft 24 and having a plurality of actuator arms 30. A spring arm 33 mounting a magnetic head 32 thereon is fixed to an end of each actuator arm 30. A coil assembly 39 having a coil 38 is fixed to the actuator assembly 26' on the side opposite to the actuator arms 30. Reference numeral 76 denotes the magnetic circuit according to the third preferred embodiment of the present invention. The magnetic circuit 76 is composed of a U-shaped main yoke 78, a pair of inner magnets 82, a pair of outer magnets 84, and a side yoke 90.

The magnetic circuit 76 will now be described in more detail with reference to Fig. 12. The main yoke 78 has an inner yoke 78a, an outer yoke 78b, and a connecting portion 78c integrally connecting the inner yoke 78a and the outer yoke 78b. The two inner magnets 82 are mounted on an outer circumferential surface of the inner yoke 78a, and the two outer magnets 84 are mounted on an inner circumferential surface of the outer yoke 78b.

The outer circumferential surface of the inner yoke 78a is formed with a pair of shoulders 79 against which circumferential opposite ends of the inner magnets 82 abut, thereby preventing circumferential slide of the inner magnets 82. The inner circumferential surface of the outer yoke 78b is formed with a pair of shoulders 81 against which circumferential opposite ends of the outer magnets 84 abut, thereby preventing circumferential slide of the outer magnets 84.

The main yoke 78 is manufactured by stacking a plurality of cold rolled steel plates 80 formed by pressing and then connecting them together by caulking. A gap 85 is defined between the inner magnets 82 and the outer magnets 84, and the coil 38 is inserted into the gap 85. An upper surface and a lower surface of each inner magnet 82 are formed with shallow grooves. Similarly, an upper surface and a lower surface of each outer magnet 84 are also formed with shallow grooves. An uppermost rolled steel plate 86 forming an uppermost layer of the main yoke 78 has a plurality of projections 86a and 86b respectively engaging with the shallow grooves formed on the upper surfaces of the inner magnets 82 and the shallow grooves formed on the upper surfaces of the outer magnets 84. Similarly, a lowermost rolled steel plate 88 forming a lowermost layer of the main yoke 78 has a plurality of projections engaging with the shallow grooves formed on the lower surfaces of the inner and outer magnets 82 and 84.

The uppermost rolled steel plate 86 further has a pair of projections 86c for downward pressing the side yoke 90, and the lowermost rolled steel plate 88 also has a pair of projections 88c for upward pressing the side yoke 90. The main yoke 78 according to this preferred embodiment is also subjected to anticorrosion treatment similar to that mentioned in the first and second preferred embodiments, so as to improve the anticorrosion property.

As mentioned above, the main yoke according to the present invention is manufactured by stacking a plurality of rolled steel plates and connecting them together by caulking. Accordingly, the main yoke having a complex shape can be manufactured simply at low costs. Furthermore, the inner magnet and the outer magnet are fixed at given positions in the main yoke without using adhesive. Accordingly, the adverse effect of the gas generating from the adhesive to the magnetic heads as in the prior art disk unit can be prevented.

 A magnetic circuit structure in a disk unit, comprising: a main yoke (78) manufactured by stacking a plurality of rolled steel plates, said main yoke having an inner yoke (78a), an outer yoke (78b) spaced a given distance from said inner yoke (78a), and a connecting portion (78c) integrally connecting said inner yoke (78a) and said outer yoke (78b) ; an inner magnet (82) mounted on an outer circumferential surface of said inner yoke (78a) opposed to said outer yoke (78b); an outer magnet (84) mounted on an inner circumferential surface of said outer yoke (78b) opposed to said inner yoke (78a); and a side yoke (90) attached to said main yoke (78) on a side opposite to said connecting portion by magnetic forces of said inner magnet (82) and said outer magnet (84); said outer circumferential surface of said inner yoke (78a) having a pair of shoulders (79) against which both circumferential ends of said inner magnet (82) abut, said inner circumferential surface of said outer yoke (78b) having a pair of shoulders (81) against which both circumferential ends of said outer magnet (84) abut. A magnetic circuit structure in a disk unit according to claim 1, characterized in that it contains a center yoke (50b) spaced a given distance from said inner yoke (50a), said center yoke (50b) being also connected by said connecting portion (50d). A magnetic disk unit having a magnetic circuit as claimed in claim 1 or 2, comprising: a magnetic disk (20) rotatably mounted in a housing (12), said magnetic disk (20) having a plurality of tracks; a magnetic head (32) for writing and reading data to and from said magnetic disk (20) ; and an actuator arm assembly (26) rotatably mounted to a base (14), said actuator arm assembly (26) having one end to which said magnetic head (32) is mounted and another end to which a coil (38) is mounted. A magnetic circuit structure in a disk unit according to claim 2, wherein each of said rolled steel plates (52) has a first surface formed with a projection (52a) and a second surface formed with a recess (52b) having a shape corresponding to that of said projection (52a), said projection (52a) and said recess (52b) of adjacent ones of said rolled steel plates (52) being engaged with each other to connect said plurality of rolled steel plates (52). A magnetic circuit structure in a disk unit according to claim 4, further comprising: at least one inner magnet fixing plate (60, 62) fixed to said inner yoke (50a) so as to project at one end portion toward said center yoke (50b), for preventing vertical slip of said inner magnet (54); and at least one outer magnet fixing plate (64, 66) fixed to said outer yoke (50c) so as to project at one end portion toward said center yoke (50b), for preventing vertical slip of said outer magnet (56). A magnetic circuit structure in a disk unit according to claim 5, wherein said inner magnet fixing plate (60, 62) comprises a pair of plates respectively fixed to an upper surface and a lower surface of said inner yoke (50a) and respectively abutting against an upper surface and a lower surface of said inner magnet (54), and said outer magnet fixing plate (64, 66) comprises a pair of plates respectively fixed to an upper surface and a lower surface of said outer yoke (50c) and respectively abutting against an upper surface and a lower surface of said outer magnet (56). A magnetic circuit structure in a disk unit according to claim 6, wherein one of said pair of plates (60, 62) abutting against said upper and lower surfaces of said inner magnet (54) has a magnet pressing portion formed by bending, and one of said pair of plates (64, 66) abutting against said upper and lower surfaces of said outer magnet (56) has a magnet pressing portion formed by bending. A magnetic circuit structure in a disk unit according to claim 4, wherein an upper surface and a lower surface of said inner magnet (54) are formed at their substantially central portions with first shallow grooves (54a); an upper surface and a lower surface of said outer magnet (56) are formed at their substantially central portions with second shallow grooves (56a) ; an uppermost rolled steel plate (70) of said main yoke (50) has a first projection (70a) projecting from said inner yoke toward said center yoke and engaging with said first shallow groove (54a) formed on said upper surface of said inner magnet (54), and a second projection (70b) projecting from said outer yoke (50c) toward said center yoke (50b) and engaging with said second shallow groove (56a) formed on said upper surface of said outer magnet; and a lowermost rolled steel plate (72) of said main yoke (50) has a third projection (72a) projecting from said inner yoke (50a) toward said center yoke (50b) and engaging with said first shallow groove (54a) formed on said lower surface of said inner magnet (54), and a fourth projection (72b) projecting from said outer yoke (50c) toward said center yoke (50b) and engaging with said second shallow groove (56a) formed on said lower surface of said outer magnet (56) . A magnetic circuit structure in a disk unit according to claim 8, wherein said uppermost rolled steel plate (70) further has a pair of projections (70c) for fixing said side yoke (58), and said lowermost rolled steel plate (72) further has a pair of projections (72c) for fixing said side yoke (58). A magnetic circuit structure in a disk unit according to claim 9, wherein said first and second projections (70c) of said uppermost rolled steel plate (70) are fixed by caulking within said first and second shallow grooves (54a, 56a) formed on said upper surfaces of said inner magnet (54) and said outer magnet (56), and said third and fourth projections (72c) of said lowermost rolled steel plate (72) are fixed by caulking within said first and second shallow grooves (54a, 56a) formed on said lower surfaces of said inner magnet (54) and said outer magnet (56). A magnetic circuit structure in a disk unit according to claim 10, further comprising a first elastic sheet interposed between said first shallow groove (54a) formed on said upper surface of said inner magnet (54) and said first projection (70a), and a second elastic sheet interposed between said second shallcw groove (56a) formed on said upper surface of said outer magnet (56) and said second projection (70b). A magnetic circuit structure in a disk unit according to claim 4, wherein said main yoke (50) has a metal coating. A magnetic circuit structure in a disk unit according to claim 4, wherein said main yoke (50) has a resin coating. A magnetic circuit structure in a disk unit according to claim 1, wherein each of said rolled steel plates has a first surface formed with a projection (6a, 86b) and a second surface formed with a recess having a shape corresponding to that of said projection, said projection and said recess of adjacent ones of said rolled steel plates (86) being engaged with each other to connect said plurality of rolled steel plates. A magnetic circuit structure in a disk unit according to claim 14, wherein an upper surface and a lower surface of said inner magnet (82) are formed with first shallow grooves; an upper surface and a lower surface of said outer magnet (84) are formed with second shallow grooves; an uppermost rolled steel plate (86) of said main yoke (78) has a first projection (86a) engaging with said first shallow groove formed on said upper surface of said inner magnet (82), and a second projection (86b) engaging with said second shallow groove formed on said upper surface of said outer magnet (84); and a lowermost rolled steel plate (88) of said main yoke (78) has a third projection engaging with said first shallow groove formed on said lower surface of said inner magnet (82), and a fourth projection engaging with said second shallow groove formed on said lower surface of said outer magnet (84). A magnetic circuit structure in a disk unit according to claim 15, wherein said uppermost rolled steel plate (86) further has a pair of projections (86c) for fixing said side yoke (90), and said lowermost rolled steel plate (88) further has a pair of projections (88c) for fixing said side yoke (90). A magnetic circuit structure in a disk unit according to claim 14, wherein said main yoke(78) has a metal coating. A magnetic circuit structure in a disk unit according to claim 14, wherein said main yoke(78) has a resin coating. Magnetkreisstruktur in einer Platteneinheit mit: einem Hauptjoch (78), das durch Stapeln einer Vielzahl von Walzstahlplatten hergestellt wird, wobei das Hauptjoch ein Innenjoch (78a), ein Außenjoch (78b), das von dem Innenjoch (78a) in einer gegebenen Distanz beabstandet ist, und einen verbindenden Abschnitt (78c) aufweist, der das Innenjoch (78a) und das Außenjoch (78b) einteilig verbindet; einem inneren Magneten (82), der auf einer äußeren Umfangsfläche des Innenjochs (78a) angebracht ist, die dem Außenjoch (78b) gegenüberliegt; einem äußeren Magneten (84), der auf einer inneren Umfangsfläche des Außenjochs (78b) angebracht ist, die dem Innenjoch (78a) gegenüberliegt; und einem Seitenjoch (90), das an dem Hauptjoch (78) auf einer dem verbindenden Abschnitt abgewandten Seite durch magnetische Kräfte des inneren Magneten (82) und des äußeren Magneten (84) angebracht ist; wobei die äußere Umfangsfläche des Innenjochs (78a) ein Paar Schultern (79) aufweist, an die beide Umfangsenden des inneren Magneten (82) stoßen, und die innere Umfangsfläche des Außenjochs (78b) ein Paar Schultern (81) aufweist, an die beide Umfangsenden des äußeren Magneten (84) stoßen. Magnetkreisstruktur in einer Platteneinheit nach Anspruch 1, dadurch gekennzeichnet, daß sie ein Mittenjoch (50b) enthält, das in einer gegebenen Distanz von dem Innenjoch (50a) beabstandet ist, wobei das Mittenjoch (50b) ebenfalls durch den verbindenden Abschnitt (50d) verbunden ist. Magnetplatteneinheit mit einem Magnetkreis nach Anspruch 1 oder 2 mit: einer Magnetplatte (20), die in einem Gehäuse (12) drehbar angebracht ist, wobei die Magnetplatte (20) eine Vielzahl von Spuren aufweist; einem Magnetkopf (32) zum Schreiben und Lesen von Daten auf die und von der Magnetplatte (20); und einem Stellgliedarmaufbau (26), der an einer Basis (14) drehbar angebracht ist, wobei der Stellgliedarmaufbau (26) ein Ende, an das der Magnetkopf (32) angebracht ist, und ein anderes Ende aufweist, an das eine Spule (38) angebracht ist. Magnetkreisstruktur in einer Platteneinheit nach Anspruch 2, worin jede der Walzstahlplatten (52) eine erste Oberfläche aufweist, die mit einem Vorsprung (52a) gebildet ist, und eine zweite Oberfläche, die mit einer Aussparung (52b) gebildet ist, die eine der des Vorsprungs (52a) entsprechende Form aufweist, wobei der Vorsprung (52a) und die Aussparung (52b) benachbarter Walzstahlplatten (52) ineinandergreifen, um die Vielzahl von Walzstahlplatten (52) zu verbinden. Magnetkreisstruktur in einer Platteneinheit nach Anspruch 4, ferner mit: mindestens einer einen inneren Magneten fixierenden Platte (60, 62), die an dem Innenjoch (50a) befestigt ist, um an einem Endabschnitt zum Mittenjoch (50b) hin vorzustehen, zum Verhindern eines vertikalen Schlupfes des inneren Magneten (54); und mindestens einer einen äußeren Magneten fixierenden Platte (64, 66), die am Außenjoch (50c) befestigt ist, um an einem Endabschnitt zum Mittenjoch (50b) hin vorzustehen, zum Verhindern eines vertikalen Schlupfes des äußeren Magneten (56). Magnetkreisstruktur in einer Platteneinheit nach Anspruch 5, worin die einen inneren Magneten fixierende Platte (60, 62) ein Paar Platten aufweist, die an einer Oberseite bzw. einer Unterseite des Innenjochs (50a) befestigt sind und an eine Oberseite bzw. eine Unterseite des inneren Magneten (54) stoßen, und die einen äußeren Magneten fixierende Platte (64, 66) ein Paar Platten aufweist, die an einer Oberseite bzw. einer Unterseite des Außenjochs (50c) befestigt sind und an eine Oberseite bzw. eine Unterseite des äußeren Magneten (56) stoßen. Magnetkreisstruktur in einer Platteneinheit nach Anspruch 6, worin eine des Paars Platten (60, 62), die an die Ober- und Unterseiten des inneren Magneten (54) stoßen, einen einen Magneten drückenden Teil aufweist, der durch Biegen gebildet ist, und eine des Paars Platten (64, 66), die an die Ober- und Unterseiten des äußeren Magneten (56) stoßen, einen durch Biegen gebildeten, einen Magneten drückenden Teil aufweist. Magnetkreisstruktur in einer Platteneinheit nach Anspruch 4, worin eine Oberseite und eine Unterseite des inneren Magneten (54) an ihren im wesentlichen zentralen Abschnitten mit ersten flachen Rillen (54a) gebildet sind; eine Oberseite und eine Unterseite des äußeren Magneten (56) an ihren im wesentlichen zentralen Abschnitten mit zweiten flachen Rillen (56a) gebildet sind; eine oberste Walzstahlplatte (70) des Hauptjochs (50) einen ersten Vorsprung (70a) aufweist, der von dem Innenjoch zum Mittenjoch hin vorsteht und in die erste flache Rille (54a) eingreift, die auf der Oberseite des inneren Magneten (54) gebildet ist, und einen zweiten Vorsprung (70b), der von dem Außenjoch (50c) zum Mittenjoch (50b) hin vorsteht und in die zweite flache Rille (56a) eingreift, die auf der Oberseite des äußeren Magneten gebildet ist, und eine unterste Walzstahlplatte (72) des Hauptjochs (50) einen dritten Vorsprung (72a) aufweist, der von dem Innenjoch (50a) zum Mittenjoch (50b) hin vorsteht und in die erste flache Rille (54a) eingreift, die auf der Unterseite des inneren Magneten (54) gebildet ist, und einen vierten Vorsprung (72b), der von dem Außenjoch (50c) zum Mittenjoch (50b) hin vorsteht und in die zweite flache Rille (56a) eingreift, die auf der Unterseite des äußeren Magneten (56) gebildet ist. Magnetkreisstruktur in einer Platteneinheit nach Anspruch 8, worin die oberste Walzstahlplatte (70) ferner ein Paar Vorsprünge (70c) zum Fixieren des Seitenjochs (58) aufweist und die unterste Walzstahlplatte (72) ferner ein Paar Vorsprünge (72c) zum Fixieren des Seitenjochs (58) aufweist. Magnetkreisstruktur in einer Platteneinheit nach Anspruch 9, worin die ersten und zweiten Vorsprünge (70c) der obersten Walzstahlplatte (70) durch Verstemmen innerhalb der ersten und zweiten flachen Rillen (54a, 56a) fixiert sind, die auf den Oberseiten des inneren Magneten (54) und des äußeren Magneten (56) gebildet sind, und die dritten und vierten Vorsprünge (72c) der untersten Walzstahlplatte (72) durch Verstemmen innerhalb der ersten und zweiten flachen Rillen (54a, 56a) fixiert sind, die auf den Unterseiten des inneren Magneten (54) und des äußeren Magneten (56) gebildet sind. Magnetkreisstruktur in einer Platteneinheit nach Anspruch 10, ferner mit einer ersten elastischen Lage, die zwischen die erste flache Rille (54a), die auf der Oberseite des inneren Magneten (54) gebildet ist, und den ersten Vorsprung (70a) dazwischengelegt ist, und einer zweiten elastischen Lage, die zwischen die zweite flache Rille (56a), die auf der Oberseite des äußeren Magneten (56) gebildet ist, und den zweiten Vorsprung (70b) dazwischengelegt ist. Magnetkreisstruktur in einer Platteneinheit nach Anspruch 4, worin das Hauptjoch (50) eine Metallbeschichtung aufweist. Magnetkreisstruktur in einer Platteneinheit nach Anspruch 4, worin das Hauptjoch (50) eine Harzbeschichtung aufweist. Magnetkreisstruktur in einer Platteneinheit nach Anspruch 1, worin jede der Walzstahlplatten eine erste, mit einem Vorsprung (86a, 86b) gebildete Oberfläche und eine zweite Oberfläche aufweist, die mit einer Aussparung gebildet ist, die eine der des Vorsprungs entsprechende Form aufweist, wobei der Vorsprung und die Aussparung benachbarter Walzstahlplatten (86) ineinander eingreifen, um die Vielzahl von Walzstahlplatten zu verbinden. Magnetkreisstruktur in einer Platteneinheit nach Anspruch 14, worin eine Oberseite und eine Unterseite des inneren Magneten (82) mit ersten flachen Rillen gebildet sind; eine Oberseite und eine Unterseite des äußeren Magneten (84) mit zweiten flachen Rillen gebildet sind; eine oberste Walzstahlplatte (86) des Hauptjochs (78) einen ersten Vorsprung (86a) aufweist, der in die auf der Oberseite des inneren Magneten (82) gebildete erste flache Rille eingreift, und einen zweiten Vorsprung (86b), der in die auf der Oberseite des äußeren Magneten (84) gebildete zweite flache Rille eingreift; und eine unterste Walzstahlplatte (88) des Hauptjochs (78) einen dritten Vorsprung aufweist, der in die erste flache Rille eingreift, die auf der Unterseite des inneren Magneten (82) gebildet ist, und einen vierten Vorsprung, der in die zweite flache Rille eingreift, die auf der Unterseite des äußeren Magneten (84) gebildet ist. Magnetkreisstruktur in einer Platteneinheit nach Anspruch 15, worin die oberste Walzstahlplatte 86 ferner ein Paar Vorsprünge (86c) zum Fixieren des Seitenjochs (90) aufweist und die unterste Walzstahlplatte (88) ferner ein Paar Vorsprünge (88c) zum Fixieren des Seitenjochs (90) aufweist. Magnetkreisstruktur in einer Platteneinheit nach Anspruch 14, worin das Hauptjoch (78) eine Metallbeschichtung aufweist. Magnetkreisstruktur in einer Platteneinheit nach Anspruch 14, worin das Hauptjoch (78) eine Harzbeschichtung aufweist. Structure de circuit magnétique dans une unité à disque, comprenant : une culasse principale (78) fabriquée en empilant une pluralité de tôles laminées, ladite culasse principale comportant une culasse interne (78a), une culasse externe (78b) espacée d'une distance donnée de ladite culasse interne (78a) et une partie de connexion (78c) connectant d'un seul tenant ladite culasse interne (78a) et ladite culasse externe (78b) ; un aimant interne (82) monté sur une surface circonférencielle externe de ladite culasse interne (78a) qui fait face à ladite culasse externe (78b) ; un aimant externe (84) monté sur une surface circonférencielle interne de ladite culasse externe (78b) qui fait face à ladite culasse interne (78a) ; et une culasse latérale (90) liée à ladite culasse principale (78) sur un côté qui fait face à ladite partie de connexion par des forces magnétiques dudit aimant interne (82) et dudit aimant externe (84); ladite surface circonférencielle externe de ladite culasse interne (78a) comportant deux épaulements (79) contre lesquels les deux extrémités circonférencielles dudit aimant interne (82) viennent en butée, ladite surface circonférencielle interne de ladite culasse externe (78b) comportant deux épaulements (81) contre lesquels les deux extrémités circonférencielles dudit aimant externe (84) viennent en butée. Structure de circuit magnétique dans une unité à disque selon la revendication 1, caractérisée en ce qu'elle contient une culasse centrale (50b) espacée d'une distance donnée de ladite culasse interne (50a), ladite culasse centrale (50b) étant également connectée par ladite partie de connexion (50d). Unité à disque magnétique comportant un circuit magnétique selon la revendication 1 ou 2, comprenant : un disque magnétique (20) monté de façon tournante dans un boîtier (12), ledit disque magnétique (20) comportant une pluralité de pistes ; une tête magnétique (32) pour écrire et lire des données sur ledit disque magnétique (20) ; et un assemblage de bras d'actionneur (26) monté de façon tournante sur une base (14), ledit assemblage de bras d'actionneur (26) comportant une extrémité sur laquelle ladite tête magnétique (32) est montée et une autre extrémité sur laquelle une bobine (38) est montée. Structure de circuit magnétique dans une unité à disque selon la revendication 2, dans laquelle chacune desdites tôles laminées (52) comporte une première surface munie d'une protubérance (52a) et une seconde surface munie d'un évidement (52b) présentant une forme correspondant à celle de ladite protubérance (52a), ladite protubérance (52a) et ledit évidement (52b) de celles adjacentes desdites tôles laminées (52) coopérant pour connecter ladite pluralité de tôles laminées (52). Structure de circuit magnétique dans une unité à disque selon la revendication 4, comprenant en outre : au moins une plaque de fixation d'aimant interne (60, 62) fixée sur ladite culasse interne (50a) de manière à faire saillie au niveau d'une partie d'extrémité en direction de ladite culasse centrale (50b), pour empêcher un glissement vertical dudit aimant interne (54) ; et au moins une plaque de fixation d'aimant externe (64, 66) fixée à ladite culasse externe (50c) de manière à faire saillie au niveau d'une partie d'extrémité en direction de ladite culasse centrale (50b), pour empêcher un glissement vertical dudit aimant externe (56). Structure de circuit magnétique dans une unité à disque selon la revendication 5, dans laquelle ladite plaque de fixation d'aimant interne (60, 62) comprend deux plaques fixées respectivement sur une surface supérieure et sur une surface inférieure de ladite culasse interne (50a) et venant en butée respectivement contre une surface supérieure et une surface inférieure dudit aimant interne (54), et ladite plaque de fixation d'aimant externe (64, 66) comprend deux plaques fixées respectivement sur une surface supérieure et sur une surface inférieure de ladite culasse externe (50c) et venant en butée respectivement contre une surface supérieure et une surface inférieure dudit aimant externe (56). Structure de circuit magnétique dans une unité à disque selon la revendication 6, dans laquelle une plaque desdites deux plaques (60, 62) venant en butée contre lesdites surfaces supérieure et inférieure dudit aimant interne (54) comporte une partie de pression d'aimant formée par flexion et une plaque desdites deux plaques (64, 66) venant en butée contre lesdites surfaces supérieure et inférieure dudit aimant externe (56) comporte une partie de pression d'aimant formée par flexion. Structure de circuit magnétique dans une unité à disque selon la revendication 4, dans laquelle une surface supérieure et une surface inférieure dudit aimant interne (54) sont formées de manière à comporter au niveau de leurs parties sensiblement centrales des premières gorges peu profondes (54a); une surface supérieure et une surface inférieure dudit aimant externe (56) sont formées de manière à comporter au niveau de leurs parties sensiblement centrales des secondes gorges peu profondes (56a) ; une tôle laminée la plus supérieure (70) de ladite culasse principale (50) comporte une première protubérance (70a) faisant saillie depuis ladite culasse interne en direction de ladite culasse centrale et s'enfilant dans ladite première gorge peu profonde (54a) formée sur ladite surface supérieure dudit aimant interne (54) et une seconde protubérance (70b) faisant saillie depuis ladite culasse externe (50c) en direction de ladite culasse centrale (50b) et s'enfilant dans ladite seconde gorge peu profonde (56a) formée sur ladite surface supérieure dudit aimant externe ; et une tôle laminée la plus inférieure (72) de ladite culasse principale (50) comporte une troisième protubérance (72a) faisant saillie depuis ladite culasse interne (50a) en direction de ladite culasse centrale (50b) et s'enfilant dans ladite première gorge peu profonde (54a) formée sur ladite surface inférieure dudit aimant interne (54) et une quatrième protubérance (72b) faisant saillie depuis ladite culasse externe (50c) en direction de ladite culasse centrale (50b) et s'enfilant dans ladite seconde gorge peu profonde (56a) formée sur ladite surface inférieure dudit aimant externe (56). Structure de circuit magnétique dans une unité à disque selon la revendication 8, dans laquelle ladite tôle laminée la plus supérieure (70) comporte en outre deux protubérances (70c) pour fixer ladite culasse latérale (58) et ladite tôle laminée la plus inférieure (72) comporte en outre deux protubérances (72c) pour fixer ladite culasse latérale (58). Structure de circuit magnétique dans une unité à disque la revendication 9, dans laquelle lesdites première et seconde protubérances (70c) de ladite tôle laminée la plus supérieure (70) sont fixées par matage à l'intérieur desdites première et seconde gorges peu profondes (54a, 56a) formées sur lesdites surfaces supérieures dudit aimant interne (54) et dudit aimant externe (56) et lesdites troisième et quatrième protubérances (72c) de ladite tôle laminée la plus inférieure (72) sont fixées par matage à l'intérieur desdites première et seconde gorges peu profondes (54a, 56a) formées sur lesdites surfaces inférieures dudit aimant interne (54) et dudit aimant externe (56). Structure de circuit magnétique dans une unité à disque selon la revendication 10, comprenant en outre une première feuille élastique interposée entre ladite première gorge peu profonde (54a) formée sur ladite surface supérieure dudit aimant interne (54) et ladite première protubérance (70a) et une seconde feuille élastique interposée entre ladite seconde gorge peu profonde (56a) formée sur ladite surface supérieure dudit aimant externe (56) et ladite seconde protubérance (70b). Structure de circuit magnétique dans une unité à disque selon la revendication 4, dans laquelle ladite culasse principale (50) comporte un revêtement métallique. Structure de circuit magnétique dans une unité à disque selon la revendication 4, dans laquelle ladite culasse principale (50) comporte un revêtement de résine. Structure de circuit magnétique dans une unité à disque selon la revendication 1, dans laquelle chacune desdites tôles laminées comporte une première surface munie d'une protubérance (86a, 86b) et une seconde surface munie d'un évidement présentant une forme correspondant à celle de ladite protubérance, ladite protubérance et ledit évidement de celles adjacentes desdites tôles laminées (86) coopérant ensemble afin de connecter ladite pluralité de tôles laminées. Structure de circuit magnétique dans une unité à disque selon la revendication 14, dans laquelle une surface supérieure et une surface inférieure dudit aimant interne (82) sont formées de manière à comporter des premières gorges peu profondes ; une surface supérieure et une surface inférieure dudit aimant externe (84) sont formées de manière à comporter des secondes gorges peu profondes ; une tôle laminée la plus supérieure (86) de ladite culasse principale (78) comporte une première protubérance (86a) qui s'enfile dans ladite première gorge peu profonde formée sur ladite surface supérieure dudit aimant interne (82) et une seconde protubérance (86b) qui s'enfile dans ladite seconde gorge peu profonde formée sur ladite surface supérieure dudit aimant externe (84) et une tôle laminée la plus inférieure (88) de ladite culasse principale (78) comporte une troisième protubérance s'enfilant dans ladite première gorge peu profonde formée sur ladite surface inférieure dudit aimant interne (82) et une quatrième protubérance s'enfilant dans ladite seconde gorge peu profonde formée sur ladite surface inférieure dudit aimant externe (84). Structure de circuit magnétique dans une unité à disque selon la revendication 15, dans laquelle ladite tôle laminée la plus supérieure (86) comporte en outre deux protubérances (86c) pour fixer ladite culasse latérale (90) et ladite tôle laminée la plus inférieure (88) comporte en outre deux protubérances (88c) pour fixer ladite culasse latérale (90). Structure de circuit magnétique dans une unité à disque selon la revendication 14, dans laquelle ladite culasse principale (78) comporte un revêtement métallique. Structure de circuit magnétique dans une unité à disque selon la revendication 14, dans laquelle ladite culasse principale (78) comporte un revêtement de résine.